1. Field of the Invention
The present invention relates to means for coating charge retaining materials with electrostatically assisted coating apparatus, in general, and to such apparatus for coating a moving web of such material, in particular.
2. Description of the Prior Art
In the manufacture of various coated products it is often essential that coating materials applied to such products be of uniform thickness. In, for example, the continuous manufacture of coated photographic sheet materials, a non-uniform thickness coating applied to a moving web of said materials may seriously interfere with the final quality of a finished product that employs such nonuniformly coated materials. Product properties such as optical, photooptical, chemical reactions (e.g., image dye migration, developer permeation, etc.), visual, aesthetic and/or cosmetic effects are but some of the said properties that may be adversely affected by non-uniform coatings. Many properties of photographic film and photographic products, for example, such as sensitivity to light, color saturation, etc., can also be adversely affected when constructed with nonuniformly coated sheet materials.
Furthermore, nonuniform coating material thickness will require considerably more drying time for drying the thicker portions of a nonuniform coating than will be required for drying the thinner portions of said nonuniform coating. In addition, a temperature gradient that is optimum for drying said thicker coating portion is often excessive for optimum drying of said thinner coating portion. Drying time is usually the major factor limiting maximum production rates of many coated products.
Various coating and/or coating application technologies have controlled the uniformity of web coating thickness to a greater or lesser degree. However, in the production of photographic film and photographic products, for example, such coating methods have a propensity for inducing surface defects, among others, in the coating material and in addition, these coating methods very often have a detrimental effect on the sensitometry of a finished photographic film product.
One of the most effective coating thickness control techniques in present day use in the coating industry involves the use of an electrostatic field to uniformly deposit coating materials on products to be coated. In the production of photographic film and photographic products, for example, a web or sheet of material to be coated is passed between an electrically conductive support or backing roller and a coating applicator from which coating material flows onto a surface of said web. An electrostatic field is established across the gap between the coating applicator and the backing roller by a high voltage power supply whose output terminals are connected between said applicator and said roller. The electrostatic field causes a coating, of uniform thickness, to be deposited on the web surface to be coated and enables larger applicator to web gaps to be employed. While the voltage magnitude established between said applicator and said roller is less than that required to generate corona, said magnitude often exceeds 3 KV DC.
The use of electrostatically assisted coating apparatus employing voltages in the vicinity of 3 KV or more can create a number of problems. In some instances voltages of this magnitude can generate sparks which would make such apparatus unsuitable for use in an explosive or solvent environment. In other instances such voltages can produce holes in the materials to be coated, thereby rendering such materials unsuitable for their intended purposes. Also, when a short circuit or extremely low impedance path appears across a coating gap between an applicator and its associated backing roller where coating material is being electrostatically assisted by a voltage of several thousand volts as a result of an existing pinhole in the material to be coated, for example, the electrostatic assist will be temporarily interrupted by said short circuit which can cause unacceptable variations in coating thickness uniformity to occur.
A web-supporting backing roller is normally maintained at a high potential by an electrostatic-field-producing high voltage power supply. This is so because its associated coating applicator is usually grounded by the coating fluid which normally is electrically conductive, to a greater or lesser degree, and said fluid provides a low impedance path to ground through its fluid-supplying conduit. However, whether it is the backing roller or the applicator that is maintained at a high potential by said high voltage power supply, a substantial risk of electrical shock is presented to personnel in the vicinity of either said applicator or said backing roller, whichever one should be connected to the high voltage power supply output lead.
It is an object of the present invention to provide electrostatically assisted coating apparatus that can place a uniform thickness of coating material on material to be coated.
It is another object of the present invention to provide electrostatically assisted coating apparatus that can be employed in an explosive or a solvent-type environment.
It is another object of the present invention to provide electrostatically assisted coating apparatus that will not produce holes in the materials to be coated.
It is another object of the present invention to provide an electrostatically assisted coating process that is immune to preexisting pinholes and/or an extremely low impedance path through material to be coated.
It is yet another object of the present invention to provide electrostatically assisted coating apparatus that will not present a shock hazard to personnel in the vicinity of said apparatus.
It is a further object of the present invention to provide electrostatically assisted coating apparatus that can tolerate a relatively large gap between a coating applicator and its associated backing roller.
Other objects and advantages of our invention will be made readily apparent by referring to the preferred embodiments of our invention described in detail below.